As with many browsers, Firefox is outdated in terms of accessibility, and text-to-speech is an accessibility feature that should most definitely be implemented. Honestly, as soon as possible.
The "Listen" capability in Reader View shows that it can be done, but this functionality should be extended to all webpages. This should honestly be standard with any browser, but it's not. Finding decent TTS software as a casual user is nigh impossible.
Many browsers' TTS capabilities are abysmal (looking at you Chrome), but Microsoft Edge has implemented this capability incredibly well. Using Immersive Reader, you only need to hit Ctrl + Shift + U to start or stop reading any web page. You can scroll through the page, and the reader will follow, or you can select text, and the reader will follow.
In Chrome, I used Speechify, which was implemented wonderfully. It even had play buttons next to individual items on the page, such as at the bottom of email drafts (to read the individual draft), next to individual Reddit comments, and next to individual paragraphs in long articles. This functionality is less essential, but is certainly quite helpful. Unfortunately, Speechify is not available as a Firefox extension, and the company has no current plans to make one. Additionally, the price of Speechify is quite steep, but I believe the popularity and willingness to pay for such a service shows that it is a widely desired functionality.
I hope I have illustrated the importance of this feature, and I hope Firefox can include such functionality soon!